I'm in the same boat, knights everywhere! So far I've found linked prisms and haywire skyweavers are getting the job done though.
I like the airwing, but I think you're right about getting more reapers. If you get up to 8-10 of them they really start to do some damage and make the most of doom and guide. 5 will do okay but as soon as they start taking casualties their effectiveness will drop off.
I'd also consider swapping the spiritseer for a warlock on bike, to make sure you'll always be in range to jinx something juicy. You could also just take a farseer on foot to save points, if he's just sitting back with the reapers.
I can't really comment on the coven stuff as I've never used them, but unless you really want 2 battalions, I'd recommend a harlequin outrider with haywire skyweavers.